For hip implants, the current standards of “hard-on-hard” bearings are ceramic-on-ceramic and metal-on-metal. The ceramics are inherently brittle in nature and are always associated with a finite risk of fracture. There are also limitations on the size of the ceramic components that can be made. Large size ceramic components (especially liners) have to be thin and may have higher risk of fracture. The metal-on-metal components have only insignificant fracture risk and larger size components can be made. The current standard material of metal-on-metal implants is high carbon Co—Cr (cobalt-chromium) alloy. The major concern with the metal-on-metal implant is the metal ion release from the joint and its unknown effects on the physiology of the human body. The advantage of metal-on-metal implants is that they can be used in larger sizes. The larger size of the implant allows greater range of motion. The metal-on-metal implants have also been shown to be useful for resurfacing types of applications where conservation of bone is desired. In such larger joints, conventional polyethylene or cross-linked polyethylene are not preferred as a counter-bearing surface and metal-on-metal is typically the only other alternative. This is due to the fact that the larger size requires a polyethylene liner to be thinner. A thinner liner may have less mechanical strength, may have increased creep, and may lead to increased wear and osteolysis and eventually to the failure of the implant.
The other commonly used hard-on-hard implant material is ceramic-on-ceramic. The current standard material of ceramic-on-ceramic implants is alumina. Metal ion release is typically not a concern for these implants. But due to limited toughness and the brittle nature of ceramics, it is difficult to make these implants in larger sizes. The ceramic components have finite probability of fracture thus leading to a potential joint failure and complications associated with the fracture of a joint.
It has been an object of much of the prior art to reduce the metal ion release and minimize the fracture risk by combining metal and ceramic components. One of the prior art approaches to reduce the risk of metal ion release is to use surface hardening of the head or liner or both using diffusion or plasma processes to incorporate nitrogen and/or carbon on the surface of the alloy. Another approach is to coat the metallic surface with ceramic coatings of nitrides (titanium nitride, chromium nitride, etc.), oxides (aluminum oxide, zirconium oxide, zirconium-alumina oxide, etc.) or diamond like carbon or diamond coatings. Another approach is to use a metal head on a ceramic liner or vice versa. In this approach, fracture risk is reduced along with the metal ion release. Another approach that has been used is the reduction of the diametrical clearance between the articulating components thereby forming a thick lubricating film which assists in the reduction of wear. Fisher et al (U.S. Patent Publication No. 2005/0033442) and Khandkar et al. (U.S. Pat. No. 6,881,229) teach the use of a metal-on-ceramic articulation. Fisher et al teach that the difference in hardness between the metallic component and the ceramic component should be at least 4000 MPa. Khandkar et. al. specifically teach use of silicon nitride ceramic components for articulation against the metallic component. In both instances, the objective is to lower the wear of mating couples. But in both instances, the fracture risk of ceramic is still significant.
Ceramic-metal composites have also seen application in the prior art. U.S. Pat. No. 6,620,523 discloses ice skating blades made from a metal matrix composite. The ice skating blade has a titanium core and a metal composite material cladding. The metal composite material may be comprised of titanium or zirconium. U.S. Patent Application Publication No. 2004/0243241 discloses an orthopaedic device, such as a spinal implant, formed of a metal matrix composite. The metal matrix composite includes a biocompatible metal alloy and a reinforcing component, such as a hard or refractory material.
However, there remains a need in the art for improved articulation joints that reduce the risk of metal ion release and the risk of monolithic ceramic fracture. There further remains a need in the art for improved articulating joints wherein one or both of the articulating surfaces are comprised of a metal matrix composite.
One particular advantage of the present invention is the reduction of the risks of fracture and metal ion release for orthopaedic implants. The risks are reduced by using a graded ceramic metal composite component with ceramic surface and graded surface below the ceramic surface. As mentioned in the details of the invention, the graded ceramic metal composite of present invention provides a solution to the above described problems pertaining to currently used hard-on-hard bearings. Although the present invention is particularly applicable to hip implants, it is also useful for orthopaedic implants generally. As examples of other embodiments of the invention, the composition described herein is applicable to knee and spinal implants and other implants wherein hard-on-hard articulation is desired.